A woman inside the school during the shooting describes "pop, pop, pop"

The sound of gunfire, confusion, a lockdown and then an evacuation. Witnesses, students and parents told frightening stories Friday about aschool shooting that police say left six adults and 20 children dead.

Many details of the attack were still unfolding, but the sight of dozens of emergency vehicles and police spread across the wooded campus made it clear Sandy Hook Elementary School has become the nation's latest infamous crime scene.

For now, evacuated children are being comforted and reuniting with their parents at a nearby firehouse where, outside, an American flag flies at half-staff.

There, couples enter and exit, some leaving without children and walking silently and tearfully past a nearby cemetery. One couple, escorted by firefighters seemed especially shaken -- eyes bloodshot and lips trembling -- stricken by the events of a clearly devastating day.

In the hours following the morning attack, parents rushed to the school after first hearing the news. They reunited with their children, clutching them and then hurrying away.

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Lynn Wasik wrapped her arms around her 8-year-old daughter, Alexis, cloaked against the cold in an oversized jacket. The girl described her ordeal after police and teachers barged into her third-grade classroom and ordered her and her classmates to hide in a corner.

"Everybody was crying," Alexis said. "And I just heard the police officers yelling."

Her mother said she first learned about the emergency through an automated phone call message. She said the message wasn't clear about the school where the incident had occurred. In a panic, she raced to Sandy Hook, eventually finding Alexis unharmed.

"My heart is in a million pieces for those families," said Lynn Wasik. "Who could do something like this? It's just sickening."

Like Wasik, other parents wrapped their arms around their children as they hurried away from the scene.

The FBI presence became much more evident in the afternoon. Several federal officers in tactical gear were coordinating with state and local law enforcement. Officers from the Bureau of Alcohol, Tobacco, Firearms and Explosives arrived to join the investigation.

"I was in the gym at the time," student Brendan Murray told CNN affiliate WABC. "I heard screaming and I thought a custodian was knocking down things. Police came in, teachers yelled to get to a safe place. Police were knocking on the doors -- police were at every door, leading us down, quick, quick."

Brendan said he later joined classmates and ran to the firehouse "really quick. We were all really happy that we were all alive."

At the firehouse, counselors such as Rabbi Shaul Praver lended a hand to help the traumatized. Some suffered from "terrible anxiety," Praver told CNN. "It's very hard to console parents in this situation," he said. "There's no theological answer to this. What you have to do is hug them and just be with them and cry with them."

Teary-eyed parents continued to emerge from the firehouse. Some were talking on cell phones, using words like "chaotic" and "devastating."

Others were openly weeping into their phones as they walked up a wooded roadway leading away from the school.

"Why? Why?" one woman cried as she walked away.

Earlier, a woman who lives near the firehouse described seeing "a bunch of children with a bunch of adults" apparently evacuating toward a nearby park. "I seen five children running up through our back field," the woman told CNN. "And by God, those poor little guys were running."

Temperatures weren't far above freezing at the time of the evacuation. Adults led children away from the school. Many of the kids wore no coats and were dressed in brightly colored clothing. They marched in a line -- one behind another -- with their hands resting on the shoulders of the children in front of them.

One parent who was inside the school at the time of the shooting described hearing a "pop, pop, pop," sound around 9:30 a.m. In the room with her were the school's principal, vice principal and the psychologist. All three left the room and went into the hall to see what was happening. The parent ducked under the table and called 911.

Later the parent said she saw two adults lying dead in the hallway, in a pool of blood.

Although school shootings have become sadly familiar in 21st century America, violence is not common in this picturesque 300-year-old town of about 27,000.

"I can't believe -- in a small town like this -- we've never had anything like this happen," a father of a Sandy Hook student told local CNN affiliate WTNH. "I was pretty shaken up. I did not know who or what happened."

"It doesn't seem possible," said another parent. "You have something happen so close to home. ... I guess I'm still in shock."

At the Stone River Grill, just a few blocks away from Sandy Hook, residents sat quietly watching TV news announce every dark detail of the tragedy.

"It's insane," said grill employee Jill Richelsoph. "This is a really nice community. We've never had anything like this." Surrounded by restaurant co-workers Richelsoph shared a conversation she had with a friend whose 5-year-old daughter attends Sandy Hook.

"She's really shaken up right now," said Richelsoph. "I don't know how she's ever going to bring her daughter back to that school."

Also at the restaurant was Tracy Ryan, who teaches at another elementary school. Newtown's schools, she said, have helped to attract new residents from different parts of the state.

"People move here for the school system," she said. "It's got great schools. I'm just totally speechless."

"My heart goes out to those families."

Just a few weeks ago Newtown -- about 60 miles outside New York City -- was recovering from Hurricane Sandy, which downed trees and knocked out power to most customers. A year before, residents suffered through Hurricane Irene.

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